In order to visualize a narrow blood vessel overlapping a thick blood vessel, at the time of performing a maximum intensity projection on three-dimensional image data, three-dimensional image data is weighted with a plurality of weighting functions of different attenuation characteristics along a projection line. Maximum intensity projections are performed on the plurality of pieces of the weighted three-dimensional image data, and results of all of the maximum intensity projections are summed. The weight of the weighting function is zero until the projection line reaches the surface of an image and, after the reach, gradually decreases from an initial value. Alternatively, the weight of the weighting function is zero to some midpoint of the projection line and, after the midpoint, gradually decreases from the initial value. The midpoint is adjustable. The attenuation characteristic is given by an exponential function. Parameters of the exponential function are adjustable.